Coated (or encapsulated) fertilizers are known to be very effective sources to provide controlled release of nutrients for the feeding of plants. The nutrients are released at controlled rates through the fertilizer's coating resulting in a sustained feeding of plants. As a result, one application of these so-called controlled release fertilizers can provide the necessary nutrients for a plant that would take multiple applications of soluble fertilizers.
These coated fertilizers may be classified into two major groups according to the fertilizer release mechanism. One type of coated fertilizer in wide use is sulfur coated fertilizer, such as disclosed in U.S. Pat. Nos. 4,042,366; 4,636,242 and 5,405,426. The release of nutrients from sulfur-coated fertilizers occurs by diffusion through imperfections in the sulfur coating and through coating breakdown. The major advantage of the sulfur coated fertilizers is their relatively low cost.
A second type of controlled release fertilizer utilizes solvent applied polymer coatings. The polymeric materials applied are either thermosetting resins or thermoplastics. Examples of solvent applied thermosetting resin coated fertilizers which are currently in use are disclosed in U.S. Pat. Nos. 3,223,518; 4,657,576 and 4,880,455; whereas examples of fertilizers having thermoplastic coatings are disclosed in U.S. Pat. No. 4,019,890. Another type of encapsulated fertilizer that exhibits good controlled release properties is latex coated granular fertilizers such as those disclosed in U.S. Pat. Nos. 4,549,897 and 5,186,732. Both solvent and latex applied polymer coated fertilizers offer important benefits over sulfur-coated products in regard to consistency of release rates. The majority of nutrient release is by diffusion through pores in the polymer coating, rather than release through coating imperfections.
Improvements to the process for producing a coated fertilizer have been investigated. For example, WO9963817 deals with the problem of incomplete or non-uniform coverage of inexpensive low quality, irregular core granules. Instead of applying a thicker layer of the encapsulating coating, this publication deals with this problem by (a) precoating the nutrient core with oil, the oil selected from the group consisting of linseed oil, soybean oil, tung oil, dicyclopentadiene modified drying oils, lubricating oils, and mixtures thereof; (b) applying an encapsulating layer of polymeric material over the precoated nutrient core, the polymeric layer being applied before the precoat is cured; and (c) curing the precoat after application of the encapsulating layer. In the examples, a commercially available NPK complex fertilizers are used as the core granules.
Fertilizers may contain different types of nutrients. The most standard fertilizer is of an NPK type. Other known secondary nutrients include calcium, magnesium and sulfur or micronutrients such as iron, copper, zinc, manganese, boron and molybdenum.
Boron is important for root development, transport of the water and minerals in plants and development of cell membranes in plants. An excess of boron gives also an issue with spots on leaves which will cause dying of the oldest leaves. A good level of boron in the soil is therefore essential. This can be provided by controlled release fertilizers containing boron.
Different combinations of a fertilizer and a coating have different types of difficulties in coating. It was found that the boron containing fertilizer granules were difficult to coat with a polyurethane coating by a standard process of applying calculated amounts of polyol and isocyanate. The granules remain wet and sticky from the reactants and the coating adheres poorly to the granules.